This invention relates to the process control industry, and particularly to a two-wire industrial process control transmitter having improved resolution without increased power requirements.
An industrial process control transmitter is a transducer that responds to a process variable and provides a standardized transmission signal that is a function of the measured variable. The term xe2x80x9cprocess variablexe2x80x9d refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH and other properties. Pressure is considered a basic process variable because it is used for the measurement of flow (difference of two pressures), level (head or backpressure), and temperature (fluid pressure in a thermal system). Industrial process pressure transmitters are used in industrial processes, such as slurries, liquids, vapors and gasses in chemical, pulp, petrochemical, gas, pharmaceutical, food and other fluid processing plants.
Many industrial process pressure transmitters are capacitor pressure transmitters that include at least two capacitor sensors formed by a deflectable sensing diaphragm and at least two capacitor electrodes, one on each side of the diaphragm. The conductive membrane deflects in response to unequal pressure applied to opposite sides of the diaphragm, the amount of deflection being based on the pressure difference. A dielectric fill-fluid fills the space between the capacitor electrodes and the diaphragm. The capacitance value of each capacitor sensor changes inversely with the distance between the respective capacitor electrode and the diaphragm. Thus the capacitances of both capacitor sensors change as the diaphragm deflects in response to applied pressures.
A charge circuit provides electrical charges to the capacitor electrodes. The charge on each capacitor is a function of the distance between the respective capacitor electrode and the conductive diaphragm, and hence is a function of the process variable being measured. The transmitter electronics includes an analog measurement circuit that measures the charge on the capacitors, an analog-to-digital converter that converts the analog signal from the measurement circuit to a binary signal, and a digital system circuit that converts the binary signal to a standardized signal for transmission to a central or control station via a two-wire communication loop. The analog measurement circuit is designed to produce an analog signal having a current that varies between 4 and 20 milliamperes (mA) over a selected pressure span of the transmitter.
Capacitor pressure transmitters are manufactured to operate within specific pressure ranges, identified by an upper range limit (URL). A pressure transmitter might, for example, have an operating range of 0 to 1,000 pounds per square inch (psi), and thus a URL of 1,000 psi. A span selection circuit selects a specific pressure range over which the 4 to 20 mA current spans within the range of the transmitter. For example, the transmitter having the URL of 1,000 psi might be operated in an environment to measure pressure span between 0 and 150 psi. The span selection circuit adjusts the detection circuit so that the maximum current (20 mA) represents the upper limit of the span, such as 150 psi for a span of 0-150 psi. The span selection circuit is adjustable to permit operation of the transmitter over several pressure spans between a minimum span and a maximum span. The maximum span includes the URL as the upper limit, whereas the minimum span is the smallest span to which the span selection circuit is adjustable. The xe2x80x9crangeabilityxe2x80x9d of the transmitter is the ratio of the maximum span to the minimum span. Thus, the transmitter having the URL of 1,000 psi and a minimum span of 0 to 67 psi has a rangeability of 1000 to 67, or 15:1.
Industrial process control transmitters are manufactured to measure pressure over various ranges and with various URLs. It is cost effective to manufacture a given model of transmitter with various URLs so that many of the transmitter parts and much of the assembly are common for the various models of the transmitter. However, the several versions of each model differ from each other due to different URLs. These versions differ from each other primarily in the construction of seals, the fill-fluid employed in the capacitor sensor, and in differences in the measurement and charge circuits. For example a Model 1151 Pressure Transmitter available from Rosemount Inc. of Eden Prairie, Minn. is available in eight different versions having URLs between 1.082 and 6,000 psi. Manufacturing costs associated with eight different versions of this transmitter could be reduced by reducing the number of ranges.
To reduce the number of models of a transmitter that are manufactured, the rangeability of the transmitter must be increased to permit selection of smaller spans. Increased rangeability requires high resolution measurement circuits.
One limiting factor on resolution resides in the power available for the transmitter circuits. Most transmitters draw power from the communication loop and are designed to draw no more than 3 milliamperes (mA) and consume no more than 18 milliwatts (mW). Present transmitter circuits employ measurement electronics that operate at 4.3 volts consuming 1.3 mA of current to produce an analog measurement signal between 4 and 20 xcexcA. The digital system electronics operate at 3.0 volts consuming. 1.7 mA of current. Thus, the transmitter draws the maximum 3 mA of current, and consumes 10.7 mW of power. At these levels, the resolution of the transmitter is about 18 bits. A reduction of the number of sensor ranges by 40% requires a digital resolution of about 24 bits, not presently available in process control transmitters with the current power distribution and analog-to-digital technologies.
In accordance with the present invention, the analog and digital portions of the analog-to-digital convertor are supported by separate dies so that noise is not induced in the analog measurement electronics by the digital system circuit. The operating power for the digital system circuit is decreased, and the power savings may be applied to the analog measurement circuit to increase resolution, and hence rangeability of the transmitter.
A two-wire industrial process control transmitter according to the present invention includes a sensor, at least two integrated circuits, and a level shift circuit. The sensor is responsive to a parameter to provide an analog signal representative of a value of the parameter. The first integrated circuit contains an analog measurement circuit that includes a sensor detection circuit and the analog portion of a digital-to-analog convertor. The analog measurement circuit is coupled to the sensor and is responsive to the analog signal from the sensor to derive a compensated analog signal representative of a value of the sensed parameter. A first power rail is coupled to a source of electric power to supply voltage at a first level to the measurement circuit on the first integrated circuit. A second integrated circuit contains a digital system circuit that includes a modem and the digital portion of the analog-to-digital convertor. The digital portion of the analog-to-digital convertor is responsive to a second compensated analog signal to provide a digital representation of the second compensated analog signal to the modem. The modem is responsive to the digital representation of the second compensated analog signal to transmit a signal representative of the digital representation. A second power rail is coupled to the source of electric power to supply a voltage at a second level to at least a portion of the digital system circuit on the second integrated circuit. A level shifting circuit shifts the voltage level of the first compensated analog signal to the second voltage level to derive the second compensated analog signal.
In one form of the invention, the transmitter is of the 4-20 mA class that draws power from a two-wire communication loop, and the analog measurement circuit and the digital system circuit together draw no more than 18 mW and no more than 3 mA from the loop. A DC-to-DC converter provides the supply voltage at the second level to the second rail and at the first level to the second rail.
In some embodiments of the invention, the analog measurement circuit is operated at a high voltage and the digital circuit is operated at a low voltage, with the high voltage being selected so that the power consumed by the analog measurement circuit is no more than 18 mW minus the power consumed by the digital circuit and the current drawn by the analog measurement circuit is no more than 3 mA minus the current drawn by the digital circuit.
In some embodiments, the digital system circuit is formed in first and second dies and a multichip module stacks the first and second dies. An additional power rail is formed in the stack so that the voltage supplied to the stack is divided among the several dies. Consequently, the portion of the digital circuit on each die is powered by a voltage difference between the module rail and the external rail. A second level shifting circuit shifts the voltage level of data signals between the dies of the stack.
In some embodiments, the analog and digital circuits are operated at different supply voltages so that maximum power is applied to the analog circuits. In other embodiments, the analog circuit and the stack of dies containing the digital circuit are operated at the same supply voltage.